Project Abstract Hundreds of studies of WTC-exposed rescue/recovery workers and others reported the incidence and prevalence of conditions that are thought to occur more commonly as a consequence of exposure to the WTC disaster site. And yet, because a suitable comparison group has not yet been identified, the most critical question remains unanswered: to what extent are apparent disease and symptom excesses associated with WTC-exposures? This project seeks to address the James L. Zadroga 9/11 Health & Compensation Act research mandate to answer critical questions about physical and mental health conditions related to the WTC terrorist attacks by enhancing our previous ground-breaking research on the WTC-exposed Fire Department of the City of New York (FDNY) firefighter cohort through the ongoing inclusion of an established comparison cohort of firefighters who did not respond to the WTC attacks. The Firefighter Comparison Cohort is comprised of career firefighters from San Francisco, Chicago, and Philadelphia who are likely to have experienced similar job-related exposures. Continued follow-up of these two cohorts (hereafter called ?the FDNY FF Cohort? and ?the FF Comparison Cohort?) offers the possibility of identifying prevention strategies for workers in the workplace and beyond. Further, we believe that maintaining the FF Comparison Cohort offers a unique opportunity to establish an excellent comparison group for a range of disaster-related research in first responders, which is a stated goal of National Institute of Occupational Safety and Health (NIOSH) leaders, and is the goal that the current proposal seeks to address. The specific objectives of this 5-year proposal are as follows: Aim 1: Determine whether firefighters in the FDNY FF Cohort have higher cancer incidence rates than firefighters in the FF Comparison Cohort. Aim 2: Establish a well-characterized cohort of firefighters from the FF Comparison Cohort for lifelong follow-up. Aim 3: Compare the post-9/11 prevalence and incidence of physical and mental health symptoms and diagnosed conditions in the FDNY FF Cohort and the FF Comparison Cohort.